The present invention relates to a superconductor for a pulsed magnet and, more particularly, to a superconductor for a pulsed magnet having a wire assembly consisting of a plurality of superconducting wires, or consisting of a plurality of superconducting wires and a plurality of normal conducting wires, wherein the wires are individually bonded by solder.
When a pulsed magnet is energized, an alternating current loss is generally produced in a superconductor constituting the pulsed magnet. In order to reduce such an alternating current loss, a pulsed magnet generally consists of a superconductor (stranded cable or braided cable) obtained by stranding or braiding a plurality of superconducting wires. In such a superconductor obtained by stranding or braiding a plurality of superconducting wires, the superconducting wires are bonded by solder with a low melting point in order to assure stable electrical and thermal characteristics of the wires and to improve the mechanical strength of the assembly.
Although solder must be used in a superconductor of the type described above, as may be seen from the above description, pulsed energization of the superconductor may result in a high coupling loss due to a coupling current induced in the solder layer. In order to reduce the coupling loss due to the coupling current, solder with high electrical resistivity must be used. In a conventional superconductor for a pulsed magnet, an Sn-Pb alloy having a re1atively high electrical resistivity is used as a solder so as to reduce the coupling current.
FIG. 1 shows the electrical resistivity .rho. (.OMEGA..multidot.cm) as a function of the Pb weight content (weight %) in the solder consisting of an Sn-Pb binary alloy. The data shown in FIG. 1 was obained by measurements made in liquid helium (4.2K) at an external magnetic field of 2 T (tesla). As can be seen from FIG. 1, the electrical resistivity of the solder consisting of an Sn-Pb alloy increases gradually until the Pb weight content in the alloy reaches about 90 wt %. However, the electrical resistivity decreases abruptly when the Pb weight content exceeds 90 wt %. The maximum electrical resistivity is as low as 5.times.10.sup.-7 .OMEGA..multidot.cm and is not satisfactory for reducing the coupling current.
In this manner, even if an Sn-Pb alloy is used for solder, a considerably high current is induced in the solder layer. Then, the coupling loss is increased, and the performance of the superconductor for a pulsed magnet is degraded. In view of this, a coupling solder is desired which has a higher electrical resistivity and which also has a low melting point and high mechanical strength at the extremely low temperatures needed for superconductivity. PG,4